The invention relates to a fault location arrangement for a digital optical transmission system, repeaters being provided between two terminal stations in the system in one or in both transmission directions, the output of each repeater being provided with an electro-optical converter coupled to the further optical transmission path, the fault location information between the repeaters and the terminal stations being conveyed by means of amplitude modulation of the optical transmission signal.
It should be noted that a repeater in such an optical system has for its object to receive the attenuated signal coming from the optical fibre, to convert it into an equivalent electric signal, to amplify it, to restore the pulse shape of the digital electric signals, to convert this restored electric signal into an equivalent optical signal, whereafter this restored optical signal is conveyed to the further optical transmission path.
The above-described method of fault location is described in, for example, United Kingdom Patent Specification No. 1,582,726. In said patent specification the fault location signal is superimposed on the digital information signal in the form of modulation of one single optical level. When, however, coherent light sources (lasers) are used in such an optical transmission system this gives rise to problems.
Owing to the amplitude modulation present, the average power dissipated in the light source will be higher in the time intervals in which the luminous flux transmitted by the light source is high than during the time intervals in which the luminous flux transmitted by the light source is low. This results in a variation of the temperature of the light source and consequently also the wavelength of the transmitted light. This effect is described in the Proceedings of the Optical Communication Conference, Sept. 17-19, 1979, Amsterdam, pages 4.2-1 to 4.2-4. Consequently a wavelength modulation of the transmitted light occurs, which modulation depends on the amplitude of the modulating signal. In an optical transmission system this wavelength modulation gives rise to the occurrence of what is commonly referred to as modal noise. Owing to the coherence of the transmitted light and the different delays associated with the mode of propagation, interference phenomena occur which results in an arbitrary fibre cross-section not being uniformly illuminated. Instead spots of light of different intensities occur. In the literature such patterns are known as speckle patterns, for example as described in Proceedings of the Fourth European Conference on Optical Communication Sept. 12-15, 1978, Genova, pages 492-501. The shape of the speckle patterns depends inter alia on the wavelength of the light. Owing to the above-mentioned wavelength modulation movement of the speckle patterns occurs. If there is a non-ideal fibre coupling somewhere in the transmission path, only a portion of the incident light, depending on a speckle pattern, will be coupled into the next fibre. As a result a transmission attenuation occurs which may vary with time, depending on the movement of the speckle pattern. By way of conclusion it may be said that the amplitude modulation of a single optical level causes an unwanted wavelength modulation of the light source which, via wavelength-amplitude conversion, will manifest itself in a parasitic amplitude modulation of the received signal.